Investigating supermassive black holes in the early Universe

Astronomers analyse star formation to detect signals from active supermassive black holes in the early Universe to better understand distant galaxy formation.

An international team of astronomers have utilised a database combining observations from some of the most advanced telescopes in the world, including the Subaru Telescope, to detect the signal from the active supermassive black holes of dying galaxies in the early Universe.

Supermassive black holes in the early Universe

The appearance of these active supermassive black holes correlates with changes in the host galaxy, suggesting that a black hole could have far-reaching effects on the evolution of its host galaxy.

The Milky Way Galaxy includes stars of various ages, including those that are still forming. However, in other specific galaxies, known as elliptical galaxies, all of the stars are old and approximately the same age.

This indicates that early in their history, elliptical galaxies had a period of prolific star formation that ended abruptly. Why this star formation ceased in some galaxies but not others is not well understood. One possibility is that a supermassive black hole disrupts the gas in certain galaxies, and creates an environment that is unsuitable for star formation.

supermassive black holes in the early Universe

The COSMOS survey region surrounded by images of galaxies used in this study. In these galaxies star formation ceased around 10 billion years ago. (3-color false-color composite images combining data from the Subaru Telescope and VISTA) (Credit: NAOJ)

Analysing distant galaxies

To test this theory, astronomers look to analyse distant galaxies. It takes time for light to travel across the void of space. For instance, the light we see from an object 10 billion light-years away had to travel for 10 billion years in order to reach Earth. Thus, the light we see today can indicate what the galaxy looked like when the light first left that galaxy 10 billion years ago.

This means that observing distant galaxies is like looking back in time. Although the intervening distance also means that distant galaxies look fainter, making observations more difficult.

To overcome these difficulties, an international team, led by Kei Ito at SOKENDAI in Japan, utilised the Cosmic Evolution Survey (COSMOS) to sample galaxies 9.5 to 12.5 billion light-years away. COSMOS combines data taken by world-leading telescopes, including the Atacama Large Millimetre/submillimetre Array (ALMA) and the Subaru Telescope. COSMOS combines radio waves, infrared light, visible light, and x-ray data.

Star formation

The team first utilised optical and infrared data to identify two groups of galaxies: those with ongoing star formation and those where star formation has stopped. The x-ray and radio wave data signal-to-noise ratio was too weak to identify individual galaxies.

As a result, the team combined the data for different galaxies to produce higher signal to noise ratio images of ‘average’ galaxies. In these images, the team confirmed both x-ray and radio emissions for the galaxies without star formation.

This is the first time that such emissions have been detected for distant galaxies more than 10 billion light-years away. Furthermore, the results highlight that the x-ray and radio emissions are too strong to be explained by the stars in the galaxy alone, indicating the presence of an active supermassive black hole.

Supermassive black hole activity

This black hole activity signal is weaker for galaxies where star formation is ongoing. These results demonstrate that an abrupt end in star formation in the early Universe correlates with increased supermassive black hole activity. More research is required to determine the details of the relationship.

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